Properties and application of polyurea
Polyurea – Beginnings
Although polyureas are still considered modern materials, they have been known for nearly 70 years. The first references to this material can be found in the Journal of Polymer Science of 1948. The journal published a list of melting temperatures of various polymers, including the title material. Over 30 years passed from the laboratory tests to the first commercial use of polyurea. In the 1980s, the development of reaction injection molding technology made it possible to manufacture components for the automotive industry and bumper claddings in Pontiac Fiero were the first manufactured elements. Polyurea as a coating has been known since 1988. The development of this technology was driven by the need for solvent-free, resilient and fast-application protective coatings. In 1989, the first commercial coating was sprayed to provide roof waterproofing.
How does it work?
Polyurea is the product of the reaction of isocyanate with polyamines. Most often, this process has two stages. The first stage usually takes place at the manufacturer’s plant and consists in the production of the prepolymer which has reactive isocyanate groups. The second stage is carried out at the processing site and consists in the reaction of the prepolymer with a mixture of polyamines, fillers and auxiliary agents. The general reaction formula is presented below:
The difference between polyurea and polyurethane is the type of functional group involved in the reaction with isocyanates. For polyurethane it is -OH and for polyurea it is -NH2. This difference significantly affects the reaction mechanism which is faster for amines. Aromatic amine is about three times more reactive than aliphatic group -OH. More importantly, the amine groups react with isocyanate much faster than with water, making the polyurea coatings moisture-resistant during processing. The gelation time for the first systems was about two seconds, and the product was cured after just ten seconds. Today, these systems are slower (gelation time is about six, curing time is about fifteen seconds). In this way, the mixture has a better flowability which significantly affects the smoothness of the coating. The polyurea applied to the water surface will cure without foaming, resulting in a tight coating. In the case of polyurethane, such spraying will not produce a tight and solid film. The systems created as results of the reaction of both discussed functional groups with isocyanate groups are called hybrid. Although the properties of polyurea are slightly better, hybrid coatings can replace them completely if less demanding components are insulated.
Aromatic or Aliphatic?
We know two basic groups of polyurea: aromatic and aliphatic. The difference between these two groups is the presence of an aromatic ring. The lack of such a ring in the polymer chain increases the UV resistance. In addition, it is often less resistant to high temperatures. Reactivity of the groups in aliphatic compounds is higher, which makes the reaction faster. A price may also be an important aspect – the decision to use an aliphatic coating will be associated with much higher costs. The following table compares other properties:
The basic equipment used for the application of polyurea is a 2-component high pressure unit. The following rules must be observed to obtain a good quality coating: Firstly, the unit must produce a pressure of min 160 and even 200 bar. Secondly, processing requires both components to be heated to approx. 70 °C. At this temperature, the viscosity of the components is low, making mixing in the nozzle more effective. Thirdly, the preparation of the substrate for spraying is just as important as the processing parameters. It shall be cleaned from any loose parts. In the case of steel, sanding must be performed. When applying to concrete, it is important to prevent the moisture from the substrate with a primer, as this will cause the coating to loosen. A fourth factor affecting the quality of insulation is the monitoring of environmental conditions. During application, the substrate temperature should be at least 3˚C higher than the dew point. Please contact your system manufacturer to receive details on polyurea.
Polyurea is an elastomeric material, so the elongation parameter is very high. At its stretchability, the material presents high tensile strength. The hardness of aromatic polyurea may not be sufficient for some applications, but the aliphatic coating has more to offer. Its excellent adhesion to many materials is worth mentioning. Other properties that distinguish polyurea from other insulations are very good crack-bridging and impact resistance. The accurate values of the basic parameters are given in the table below:
Pipe protection – polyurea is ideal for protecting pipes and pipelines. It protects them against corrosion and external influences. It can be applied to both steel and polyurethane foam which is the thermal insulation of the pipeline.
Bridges and tunnels – the ability to insulate both steel and concrete, as well as high durability allow to secure structures without the need for renovation for many years (ageing tests indicate that the coating can fulfil its function even for 30 years).
Tank protection – for both new and refurbished tanks. The tank can be put back into service within 48 hours. The material provides very good chemical and mechanical resistance.
Maritime industry– applications are not just about protecting steel and laminates: polyurea is often used for soundproofing hulls and hold linings.
Roof insulation – mechanical strength, durability and speed of application make polyurea irreplaceable in some uses. Systems with increased reflectivity that protect, for example, storage halls from overheating are increasingly popular.
Sewerage and water supply systems – very wide range of applications. From covering manhole elements, through sewage treatment plants, to pipeline repair systems. It is very important that the element requires downtime for a short period. The trenchless repair of pipelines using polyurea is also a very interesting technology.
Automotive industry – initially it was mainly a protective layer on a car platform, now it is a coating of the whole body protecting against corrosion, or in the case of off-road cars – against mechanical damages.
Of course, these are just a few examples of applications. In addition, among the less popular uses we can find aquarium insulation, sports surfaces, foundation insulation, armouring, bulletproof glass or loudspeaker coatings.